Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element modules including optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units. Further components of such optical systems are aperture devices defining the geometry of the light beam used in the exposure process.
Due to the ongoing miniaturization of semiconductor devices there is a desire for enhanced resolution of the optical systems used for fabricating those semiconductor devices. One possibility to enhance resolution of the optical systems is to reduce the wavelength of light used in the exposure process. Thus, there currently is a strong tendency to use light in the so-called extreme UV (EUV) range at wavelengths between about 5 to 20 nm, typically at about 13 nm. However, in this EUV range the use of refractive optical element may not be possible any more due to the high absorption of light of such a short wavelength within any medium, in particular within refractive optical elements.
Consequently, not only exclusively reflective optical systems are used in such an EUV system but also a highly evacuated atmosphere has to be maintained within the part of the light used in the exposure process. Due to the limitations in increasing the radiant power of the EUV light fed into the optical exposure system (e.g. due to increasingly hard to handle heating effects etc) and the extremely high sensitivity of the EUV light to absorption care has to be taken to lose as few radiant power as possible all the way down to the substrate to be exposed.
One problem arising in this context is the loss of radiant power at aperture devices located within the path of the exposure light. In some cases, for example, it may be favorable (e.g. for a sigma variation) to have an aperture device which is able to provide variable annular settings. Typically, in non EUV systems, such annular settings are provided by a solid aperture plate (e.g. made of quartz) having a shielding outer section and a shielding inner section separated by a ring shaped transparent section allowing the light to pass the aperture. However, in an EUV system such an aperture plate may obviously not be used any more for reasons of absorption.
A solution could be to provide the aperture plate with corresponding cutouts in the region of the aperture such that only a few strut elements remain to radially connect the inner shielding section to the outer shielding section. However, such radial struts typically could also obstruct the path of the light through the aperture leading to an unwanted loss in radiant power.
A further problem arises in the context of providing varying annular settings with such EUV systems. A variation of the annular setting typically also includes a variation of the inner contour delimiting the annular aperture. Typically, in non EUV systems, such a variation of the annular setting is provided by exchanging the aperture plate with a different aperture plate providing the desired setting. However, in an EUV system (with its strict desired properties with respect to the high degree of evacuation and the low degree of contamination of the atmosphere in the path of the exposure light) such an exchange of a rather large and bulky structure can represent a considerable challenge with respect to the complexity and accuracy of the handling mechanism, the maintenance of a high-quality atmosphere (high vacuum, low contamination etc) and the space involved.
It will be appreciated that a solution to the above problems in an EUV system would also be suitable and beneficial in a conventional non EUV system.